A developing source of energy is subterranean water sources which are heated by the earth's magma and which are otherwise known as geothermal brines. One way of extracting energy from geothermal brines is to transfer heat from the geothermal brine directly to a working fluid by means of a heat exchanger. The working fluid is used to produce work as, for example, when it expands through a turbine. Thus, corrosion and scale deposits on turbine blades and other turbine components, which are powered by the working fluid, is avoided as the geothermal brines, which usually contain dissolved solids, are not used directly therein.
One type of heat exchanger, the flash-type evaporator direct contact, binary fluid geothermal heat exchanger or boiler operates by the direct contact between two immiscible fluids, one being the geothermal brine, and the other being a working fluid such as, for example, isobutane. In the flash-type heat exchanger a liquid or mixture of liquids which serves as the working fluid is initially pressurized to prevent boiling and is heated to a temperature above its saturated temperature at the desired final pressure. When the over pressure is relieved a equilibrium portion of the liquid flashes into vapor.
Several problems are associated with the direct contact, binary fluid geothermal heat exchangers. One of these problems is the efficiency of heat transfer between the geothermal brine and the working fluid. The heat transfer co-efficient for a prior art heat exchanger 20 as depicted in FIG. 1 is unnecessarily reduced due to the particular apparatus which is used to introduce the liquid brine into the liquid working fluid, which in this case can be isobutane. As can be seen in FIG. 1, the working fluid enters the lower portion of the heat exchanger through a port 22 and is provided through distributor 24 in the form of droplets 25. The geothermal brines are introduced through port 26 and directed to distributor 28. As the isobutane has a specific density which is less than the geothermal brines, and as the isobutane is immiscible with the geothermal brines, the droplets of isobutane flowed upwardly in the geothermal brines. Due to the flow rate and other factors which will be discussed hereinbelow, the level of the mixture of geothermal brines and isobutane, in both liquid and gaseous phases, is determined and is indicated at 30. Immediately above the top of this brine-continuous zone (wherein isobutane is distributed in the brine) is a transitional zone which essentially divides the liquid and gaseous mixtures of geothermal brines and isobutane from a vaporous-continuous zone which exists thereabove in area 32. The transitional zone which is indicated at 34 is comprised of a foaming, frothing and boiling area of liquid and gaseous geothermal brines and isobutane.
As can be seen in FIG. 1 distributor 28 extends well below this transitional zone 34. Accordingly, liquid geothermal brines are introduced directly into contact with liquid isobutane through ports 36. Such liquid-liquid contact offers great resistance to heat transfer and thus has a low heat transfer coefficient. The reason for this low heat transfer coefficient can be seen more clearly in FIG. 1A. As the brine is introduced into the isobutane, the isobutane immediately adjacent thereto vaporizes and forms an isobutane, vaporous blanket 38 about a droplet of liquid isobutane 40. Consequently the only area where good heat transfer can occur is where the isobutane droplet 40 contacts the surface of the isobutane blanket 38 at point 42, the lowest point thereof. Stated alternatively, frothing is produced when the isobutane is vaporized beneath level 30 by the hot brines, forming a metastable cellular structure of isobutane bubbles in the brines whereby heat transfer to the isobutane droplets inside the bubbles is stiffled by the low conductivity of the isobutane gas blanket. The isobutane droplet and the accompanying blanket 38 must rise to level 30, before the blanket is disbursed and the droplet vaporized. Accordingly below level 30 only a preheat zone exists, and not an efficient isobutane vaporization or flash zone with a high heat transfer coefficient.
Because of the submerged introduction of the geothermal brines in the liquid isobutane, and the associated flashing of the isobutane there is a considerable amount of back-mixing beneath the level 30. This back-mixing can propagate wave fronts vertically in the heat exchanger 20 with resultant instabilities and level surges in the operation thereof and in particular in the size and location of transition zone 34 and the position of level 30. Such instability in the level 30 can cause froth which includes liquid and vaporous geothermal brine to carry over through isobutane outlet port 44 and into the turbine, causing scaling and corrosion of said turbine. A demister 46 placed at the upper end of gaseous area 32 will generally not be adequate enough to stop the brine mist from entering the turbine.
Another problem with the present heat exchanger 20 is that in order to increase the capacity of a system using exchanger 20 a plurality of such exchangers would have to be incorporated into the system as increasing the size of boiler 20 beyond a certain range would not produce a proportional increase in the transfer of heat to the isobutane.
In another type of prior art heat exchanger 50 is depicted in FIG. 2. Elements in boiler 50 which are similar to those in the boiler 20 of FIG. 1 are identified with similar numerals, which have been primed. Boiler 50 differs from boiler 20 in that the geothermal brine distributor 52 includes a distributor head 54 which has a plurality of ports for allowing the geothermal brines to rain down upon level 30' and zone 34'. The brine, besides causing the isobutane to flash to vapor upon direct contact therewith, also itself flashes to vapor and atomizes as it leaves the distributor head 54. Consequently there is a mixture of brine and isobutane vapors in gaseous area 32' and accordingly, excessive brine vapor and mist carry-over through outlet port 44' into the turbine even with the addition of demister 46'.
The uncontrolled flashing instabilities and surges throughout the heat exchangers of FIGS. 1 and 2 also promote loss of isobutane from the bottom of the boiler through brine outlet ports 48 and 48'.
Accordingly it is an object of the present invention to provide a controlled means for boiling a working fluid for extraction of geothermal energy by direct contact with hot, pressurized geothermal brines.
Another object is to provide direct contact between the geothermal brines and the working fluid in such a manner as to provide a controllable amount of superheat in the working fluid vapor, so that the boiling of the two immiscible fluids can be made to occur very close to equilibrium conditions as predicted by the Gibbs Phase Rule.
Another objective of the present invention is to provide for direct contacting of the geothermal brine and the working fluids so as to minimize entrainment of the liquid geothermal brines in the isobutane vapors which are removed from the heat exchanger and carried over into the turbine causing corrosion and scaling thereof.
Another object of the present invention is to minimize back-mixing in the geothermal heat exchangers in order to stabilize the transition zone and to additionally prevent the carryover of entrained geothermal brines into the turbine.
Still another object of the present invention is to increase the heat transfer efficiency in the heat exchanger.
Another object is to design a single heat exchanger to replace the plurality of heat exchangers which are presently required in large capacity systems.